2,6-DMN is an intermediate produced during the manufacture of 2,6-naphthalene dicarboxylic acid (2,6-NDA) and 2,6-naphthalene dicarboxylate (2,6-NDC). 2,6-NDA and 2,6-NDC are monomers either of which, when combined with ethylene glycol, reacts to make polyethylene naphthalate (PEN), a polyester with unique and advantageous commercial applications in films, fibers, and packaging.
The isomers of dimethylnaphthalene are difficult to separate from one another by distillation because their boiling points are very similar. Technology exists to recover 2,6-DMN by crystallization, or by adsorption, or by adsorption followed by crystallization. It is difficult to separate 2,6-DMN from 2,7-DMN by crystallization alone because they form a eutectic. It is expensive to recover 2,6-DMN from mixed DMNs by adsorption alone because there are no known materials that selectively adsorb 2,6-DMN. In previous technology, when adsorption and crystallization steps were combined, the adsorption step was always used to remove the majority of the undesired DMN isomers.
Furthermore, the adsorption step was often followed by an additional crystallization step to obtain the desired product purity.
An additional complication in the commercial production of 2,6-DMN is the difficulty of converting DMN isomers other than 2,6-DMN into the desired 2,6-DMN isomer. It is well known that during DMN isomerization, it is easy to move methyl groups on a naphthalene ring when they migrate from an alpha position (i.e., 1, 4, 5 or 8) to a beta position (i.e., 2, 3, 6 or 7) or vice versa, but it is difficult when the methyl groups must rearrange from one beta position to another. DMN isomers have been classified into groups called "triads" within which isomerization is readily accomplished. These triads are (1) 1,5-DMN, 1,6-DMN, and 2,6-DMN; (2) 1,7-DMN, 1,8-DMN, and 2,7-DMN; and (3) 1,3-DMN, 1,4-DMN, and 2,3-DMN. The tenth isomer, 1,2-DMN, consists of two methyl groups in adjacent alpha and beta positions and does not fall into one of the aforementioned triads.
Producers have developed methods for making commercial quantities of 2,6-DMN by avoiding co-producing the 2,7-DMN isomer because of the difficulty in recovering 2,6-DMN at high yield in the presence of 2,7-DMN. Furthermore, producers have tried to avoid making isomers outside the 2,6-triad because of the difficulty in isomerizing across triads. Isomers that cannot be converted to 2,6-DMN represent a yield loss and inefficient use of raw materials. Additionally, adsorption is not practical when the concentration of 2,6-DMN in the feed stream is low because there are no known materials that will preferentially adsorb 2,6-DMN over the other isomers. These limitations often necessitate the use of expensive raw materials and controlled organic synthesis reactions that can produce only isomers in the 2,6-triad, such as alkylation of butadiene and ortho-xylene, and methylation of methylnaphthalene.
Technologies relating to the purification of 2,6-DMN from DMN isomer mixtures by crystallization, adsorption and distillation are known as are technologies relating to the isomerization of non-2,6-DMN to 2,6-DMN.
Separation of DMN isomers by crystallization is relatively straightforward if the feed composition is quite high in 2,6-DMN isomer, or if a low yield is acceptable, or if the feed to be crystallized consists of isomers within a triad. If the concentration of 2,6-DMN is well above the eutectic composition, simple crystallization can produce pure 2,6-DMN in high yields. If the concentration of 2,6-DMN is slightly above the eutectic composition, low yield of high purity DMN can be obtained. If the mixture consists of isomers within the 2,6-DMN triad, the unrecovered material, a mixture of 1,5-DMN, 1,6-DMN and 2,6-DMN, can be easily isomerized to produce a mixture with 2,6-DMN above the eutectic composition. Crystallization alone becomes insufficient to purify mixed DMNs to make 2,6-DMN when both the 2,6-DMN and 2,7-DMN isomers are present because they form a eutectic.
The feasibility of adsorption separation for DMN isomers has been demonstrated. However, no material has been published that selectively adsorbs 2,6-DMN from a feed of mixed DMN's. This limitation makes it expensive to recover 2,6-DMN from mixed DMNs by adsorption alone because the adsorption equipment must be very large in order to remove all components other than 2,6-DMN from a feed stream that contains small quantities of 2,6-DMN.
One technique for overcoming the limitations of 2,6-DMN purification by crystallization or by adsorption is to combine the two technologies. Such a combination has always previously been done by using the adsorption step as a feed pretreatment step prior to the crystallization step.
An alternative technique to break the 2,6-DMN/2,7-DMN eutectic is to partially or completely saturate the naphthalene ring. The resulting decalins or tetralins do not form a eutectic at the same composition as the dimethylnaphthalenes, so an incremental quantity of the 2,6- and 2,7-isomers can be recovered by alternately hydrogenating and dehydrogenating the DMN feed.
It has been disclosed that a noneutectic DMN mixture containing 2,6-DMN and 2,7-DMN along with smaller amounts of other hydrocarbons can be sublimated so that the remaining solid is a mixture of 2,6-DMN and 2,7-DMN. However, no teaching is given that sublimation can be used to purify a 2,6-DMN/2,7-DMN mixture which is in the form of an eutectic composition.
Previous isomerization technologies have been limited to intra-triad conversions, i.e., movement of methyl groups between adjacent alpha and beta positions only. Santilli and Chen, U.S. patent application Ser. No. 08/892,508, filed Jul. 14, 1997, now U.S. Pat. No. 6,015,930, which is incorporated herein by reference, discloses a method of isomerizing a feed of any composition of mixed dimethylnaphthalenes having a methyl group on each ring to a product that approximates an equilibrium mixture of mixed dimethylnaphthalenes having a methyl group on each ring (i.e., the 2,6-DMN and 2,7-DMN triads). The method of the present invention incorporates this method of isomerization across the two triads.
Researchers have integrated separation and isomerization technologies in an attempt to improve the overall process of 2,6-DMN production. These various attempts to integrate the technologies have had limited success because the various steps of the process suffer from such problems as low yields or the inability to isomerize between triads.
The technologies discussed above relate either generally or specifically to certain aspects of the presently claimed invention. These technologies are either not very effective or not economical for obtaining substantially pure 2,6-DMN from feeds containing a variety of DMN isomers outside the 2,6-triad. What is needed is an economic method to produce 2,6-DMN at high purity and high yield from a mixture of DMN isomers without being limited to the specific isomers present in the feed. A new method should convert isomers other than 2,6-DMN into the desired 2,6-DMN isomer in order to have an acceptable yield of 2,6-DMN from the feed source. The present invention accomplishes these goals.